1. Field of the Invention
The invention relates generally to antennas and in particular relates to multisided horn antennas.
2. Prior Art
Horn antennas for transmitting and receiving microwave energy are generally known in the prior art. In numerous satellite applications, horn antennas with square apertures are usually used for a variety of reasons including, for example, to provide a closely packed array of horns with greater efficiency. Additionally, a square horn antenna has an aperture with greater area than a conical horn antenna of the same diameter. Therefore, greater power may be radiated from a given area of an antenna array. There are, however, several drawbacks associated with square horn antennas, the principal drawback being that the axial ratio or ellipticity is inferior to that of a conical antenna or similar aperture shape. The engineering "trade-off" of a square horn antenna is its capability of transmitting greater power at the sacrifice of ellipticity. The ellipticity of small square horn antennas has not, prior to the present invention, been substantially improved upon without a significant sacrifice in efficiency. One of the techniques has been to insert a dielectric ring or sleeve in the throat of the horn. Such a dielectric ring must be accurately placed so that the impedance between the ring loaded launcher and the flared section of the horn is matched.
Another method of improving the ellipticity of a square horn antenna is with the use of corrugations within the horn. These corrugations excite the higher order modes and thereby improve ellipticity. The corrugation technique requires relatively large apertures thereby requiring a relatively large horn and the attendant increase in weight. Such an increase in weight has the distinct disadvantage in satellite applications. Also, the corrugated horn is usually machined from a solid metal stock and the corrugations must be precisely machined. Consequently, cost of machining a corrugated horn is high.
Fins have also been employed in order to equalize E and H fields, and thereby improve ellipticity at the aperture of horn antennas. Usually two sets of fins are placed in diametric opposition providing E and H symmetry to the signal from the horn. The size, number and locations of the fins are critical for providing phase compensation to the two fields. Using fins, however, provides poor isolation between orthogonal waves such as right and left-hand circularly polarized waves being propagated within a square horn antenna.